What is the ZF 4-speed conversion in a Triumph Stag?

It is the replacement of the original 3-speed Borg-Warner 35 automatic with a ZF 4HP22 four-speed automatic transmission. The ZF adds a fourth, overdrive-style top gear and a lock-up torque converter, which lowers cruising engine speed on the motorway and reduces heat and noise. It is a well-established conversion within the Stag community, usually undertaken as a bell-housing adaptor package with modified propshaft, mounts and cooler lines.

Why upgrade from the Borg-Warner 35 to a ZF 4HP22?

The BW35 is a robust unit but was designed as a three-speed with no overdrive top and no torque converter lock-up. At UK motorway speeds the Stag V8 sits at a relatively high rpm, which increases engine noise, heat and fuel consumption. The ZF 4HP22 gives a taller cruising gear, quieter and cooler motorway running, and generally crisper part-throttle response.

Is the ZF conversion reversible?

In principle yes. The Stag's engine block, bell-housing face and gearbox tunnel are not permanently altered by a good conversion — the BW35 can be refitted later using its original mounts, propshaft and linkage. Keep the original gearbox, propshaft, cooler pipes and selector parts if originality or resale matter to you.

Does the ZF 4HP22 affect originality or value?

For a concours or matching-numbers car it is a deviation from factory specification and should be declared. For a driver-focused Stag used regularly on modern roads, most owners regard a well-executed ZF conversion as a sympathetic upgrade rather than a demerit. The market treats it similarly to a fitted overdrive on a manual car — welcomed by users, noted by purists.

How much does a ZF 4-speed conversion cost?

Costs vary widely with the source of the gearbox, the specific conversion kit, and how much work the car needs around it (cooler, propshaft, mounts, wiring, selector). Treat published figures as guidance only and obtain a written quote from a Stag specialist before committing.

Ownership / ZF 4-Speed Automatic Conversion

Triumph Stag ZF 4-Speed Automatic Conversion

Adding a taller top gear and lock-up torque converter to an automatic Stag.

The ZF 4HP22 conversion replaces the Stag's original 3-speed Borg-Warner 35 automatic with a modern four-speed automatic that has a taller overdrive top and a lock-up torque converter. The result is noticeably lower motorway rpm, a cooler and quieter drive, and better long-distance manners — while keeping the character of an automatic Stag.

General guidance for the Triumph Stag, not a workshop specification. Talk to a Stag specialist about a specific conversion kit for your car.

Section 1

Why owners do it

The Borg-Warner 35 was a well-regarded 1960s three-speed automatic and it suits the Stag's V8 at town and A-road speeds. Its limitation is at the top end: without an overdrive fourth and without a lock-up torque converter, the engine sits at a fairly busy rpm on the motorway. That means more noise in the cabin, more heat into the transmission and the engine bay, and higher fuel consumption than the car really needs.

The ZF 4HP22 addresses both points directly. Fourth gear is an overdrive-style ratio that drops cruising rpm significantly, and the lock-up torque converter eliminates slip once you are settled at a steady speed — the drivetrain effectively behaves like a direct mechanical connection instead of a fluid coupling. On a long run the difference in refinement is usually the first thing owners notice.

Section 2

Borg-Warner 35 vs ZF 4HP22 — at a glance

Feature	Borg-Warner 35 (original)	ZF 4HP22 (conversion)
Forward gears	Three	Four (with overdrive top)
Torque converter lock-up	No	Yes, in top gear
Motorway cruising rpm	Higher — busier engine	Lower — noticeably calmer
Refinement at speed	Period feel	Modern grand-tourer feel
Heat into the transmission	Higher — always some converter slip	Lower once locked up
Parts & service support	Specialist rebuild only	Widely used in period BMW/Jaguar etc.
Originality	Factory specification	Sympathetic upgrade — declare on sale

Section 3

What changes on the car

A ZF 4HP22 conversion in a Stag is typically supplied as a package rather than a single part. The exact scope depends on the kit and installer, but most conversions involve some combination of the following:

Adaptor arrangement so the ZF bolts to the Stag V8's bell-housing face and picks up the crankshaft correctly.
Modified or new propshaft to suit the ZF's different length and output flange.
Gearbox mount and cross-member reworked to carry the ZF in the Stag's tunnel.
Cooler pipework to and from the radiator or a dedicated auxiliary transmission cooler.
Selector linkage adapted so the original quadrant still reads correctly (P-R-N-D-3-2-1 or similar), or a new quadrant fitted.
Lock-up control, either the ZF's own hydraulic scheme or a simple electrical control fitted during installation.

None of the above requires cutting the Stag's chassis rails or engine block. A well-executed conversion should leave the car mechanically reversible to BW35 specification later.

Section 4

Cooling and fluid — do not skip this

Automatic transmissions live or die on their oil temperature. The ZF is no exception. Fit a properly sized transmission cooler, use the manufacturer-specified ATF, and keep the Stag's engine cooling system in good order — see the cooling upgrades guide for the wider engine-side picture. A ZF gearbox running on tired fluid or hot coolant will not last, no matter how well the conversion was installed.

Section 5

Trade-offs and originality

The ZF conversion is not a magic wand. It costs real money, it takes the car off the road while it is done, and it is a deviation from factory specification that should be disclosed on sale. For a matching-numbers concours car, that alone may rule it out. For a Stag used as a modern long-distance tourer, most owners who have done the conversion regard it as one of the best-value refinement upgrades available — comparable to adding overdrive to a manual car.

If you are unsure, keep the original BW35, propshaft and selector parts when the conversion is done. That preserves the option of returning the car to factory specification later, and gives a future owner the choice.

Common questions

FAQ

What is the ZF 4-speed conversion in a Triumph Stag?: It is the replacement of the original 3-speed Borg-Warner 35 automatic with a ZF 4HP22 four-speed automatic transmission. The ZF adds a fourth, overdrive-style top gear and a lock-up torque converter, which lowers cruising engine speed on the motorway and reduces heat and noise. It is a well-established conversion within the Stag community, usually undertaken as a bell-housing adaptor package with modified propshaft, mounts and cooler lines.
Why upgrade from the Borg-Warner 35 to a ZF 4HP22?: The BW35 is a robust unit but was designed as a three-speed with no overdrive top and no torque converter lock-up. At UK motorway speeds the Stag V8 sits at a relatively high rpm, which increases engine noise, heat and fuel consumption. The ZF 4HP22 gives a taller cruising gear, quieter and cooler motorway running, and generally crisper part-throttle response.
Is the ZF conversion reversible?: In principle yes. The Stag's engine block, bell-housing face and gearbox tunnel are not permanently altered by a good conversion — the BW35 can be refitted later using its original mounts, propshaft and linkage. Keep the original gearbox, propshaft, cooler pipes and selector parts if originality or resale matter to you.
Does the ZF 4HP22 affect originality or value?: For a concours or matching-numbers car it is a deviation from factory specification and should be declared. For a driver-focused Stag used regularly on modern roads, most owners regard a well-executed ZF conversion as a sympathetic upgrade rather than a demerit. The market treats it similarly to a fitted overdrive on a manual car — welcomed by users, noted by purists.
How much does a ZF 4-speed conversion cost?: Costs vary widely with the source of the gearbox, the specific conversion kit, and how much work the car needs around it (cooler, propshaft, mounts, wiring, selector). Treat published figures as guidance only and obtain a written quote from a Stag specialist before committing.